The invention relates to a compression-ignition internal combustion engine, in which fuel is injected into a combustion space by an injection nozzle having a plurality of injection bores.
In direct-injection internal combustion engines with compression ignition, for the configuration of a load-dependent fuel injection, injection nozzles are used in which the configuration of the injection profile is controlled by a corresponding construction of the injection nozzle. This is intended to improve mixture preparation in the combustion space and therefore the formation of emissions in the exhaust gas. Furthermore, the functioning of downstream exhaust gas treatment can be optimized.
DE 19953932 A1 discloses a method in which a combined homogeneous/heterogeneous type of operation of an internal combustion engine for the achievement of medium and higher powers is proposed. In this context, both early homogeneous mixture formation in the compression stroke and subsequent heterogeneous mixture formation around top dead center are to become possible by an injection strategy, fuel injection taking place at a lower injection pressure during homogeneous mixture formation than during heterogeneous mixture formation, in order to avoid fuel being applied to the cold combustion space walls. Experience has nevertheless shown that, in spite of the measures proposed above, increased exhaust gas emissions continue to occur. Further measures, by which the exhaust gas emissions are minimized, must therefore be taken.
The object on which the invention is based is to provide an internal combustion engine with compression ignition, by which mixture formation and combustion in the combustion space are improved. This is achieved, according to the invention as follows.
The internal combustion engine according to the invention is distinguished in that fuel is injected, by an injection nozzle having a nozzle needle and possessing a plurality of injection bores, into a combustion space formed between a piston and a cylinder head, in the form of a plurality of fuel jets as a main injection and, if appropriate, as a preinjection and/or postinjection, the injection bores of the injection nozzle being arranged in at least two different separately activatable rows of holes, and an operating stroke of the nozzle needle being capable of being set by the control unit as a function of a piston position (φ) and/or of an operating point of the internal combustion engine (1).
Accordingly, a controlled injection of the fuel for varying the jet length in the combustion space can be carried out as a function of the piston position in the cylinder, with the result that the intermixing of the injected fuel with the combustion space air, particularly during preinjection and postinjection, takes place before the cylinder wall is reached. A wetting of the cylinder wall with fuel can thereby largely be avoided.
According to a refinement of the invention, the rows of holes of the injection nozzle have different injection-hole cone angles. The fuel can thereby, in particular, be introduced into the combustion space at a steeper injection angle during a preinjection or during a postinjection than during the main injection. The jet length in the combustion space can thus be adapted as a function of the distance between the injection nozzle and the piston, so that a variation in the jet length in order to optimize the mixture formation is thereby ensured or made possible.
In a further refinement of the invention, a first row of holes of the injection nozzle is activated during main injection and a second row of holes is activated during preinjection and/or postinjection, the injection-hole cone angle of the first row of holes being higher than the injection-hole cone angle of the second row of holes. In this case, during the main injection, the fuel is injected around a top dead center through the first row of holes at a flat injection-hole cone angle, preferably of between 140° and 160°. By contrast, in the event of early and/or late fuel injection, the fuel is injected into the combustion space at a steep injection cone angle, for example of between 60° and 160°, through the second row of holes arranged preferably below the first row of holes. This makes it possible to have an optimum jet length in the combustion space during an injection operation. According to the invention, depending on the operating point, an injection operation comprises a main injection and, as required, a preinjection and/or postinjection.
According to a further refinement of the invention, a fuel injection pressure can be set as a function of the piston position (φ) and/or of an operating point. As a result, furthermore, the jet length of the fuel introduced into the combustion space can be influenced.
In a further refinement of the invention, an operating stroke of the nozzle needle of the injection nozzle can be set in such a way that an unstable cavitating flow is formed in the injection bores of the injection nozzle. Preferably, during the preinjection and/or postinjection, the fuel is injected intermittently in the form of small part quantities. Accordingly, owing to the variable setting of the operating stroke of the nozzle needle of the injection nozzle, in the case of each preinjection or postinjection part quantity injected into the combustion space, the atomization of the respective fuel jet in the combustion space can be reinforced, so that a wall accretion of fuel in the cylinder, which, for example, rises continuously in the case of a lower gas pressure and lower temperature in the cylinder, is minimized. Thus, according to the invention, the range of each part quantity up to a combustion space wall is limited and an intensified break-up and evaporation of the injection jet or of the part quantity, particularly with an increasingly later start of injection, are achieved.
According to a further refinement of the invention, a swirl movement can be set in the combustion space of the internal combustion engine. Preferably, a generated fuel cloud of a fuel jet (17) is offset or laterally displaced by the swirl movement set in the combustion space, in particular during a fuel injection carried out intermittently. As a result, for example, the formed fuel cloud of a part quantity, when it penetrates through the combustion space, can be laterally displaced, in particular in the direction of a cylinder wall. The fuel jets or the fuel droplets are thus moved away or moved further from the cylinder wall during their propagation, so that they are intermixed with the combustion space air or evaporate, in particular during preinjection and postinjection, before they reach the cylinder wall. A wall accretion of fuel in the cylinder is thus largely prevented. If there is no swirl movement in the combustion space, the fuel jet is propagated along an injection-hole center axis and impinges onto the cylinder wall on account of the short travel up to the cylinder wall. Furthermore, in this case, the preceding jet parcels or the jet parcels emerging first from the injection nozzle form a jet duct which results in an accelerated penetration of the following jet parcels or part quantities due to a lee effect, so that, in the absence of a swirl movement of the combustion space, an impingement of fuel onto the cylinder wall becomes more likely.
In a refinement of the invention, the operating stroke of the nozzle needle is set in such a way that, within the injection nozzle, an effective flow cross section between the nozzle needle and a nozzle needle seat amounts to about 0.8 to 1.2 times an effective flow cross section of the sum of all the injection bores. A desired throttling action can thereby be achieved in a controlled way in the seat of the nozzle needle and gives rise to an unstable cavitating flow in the injection bores of the injection nozzle.
In a further refinement of the invention, the piston has a piston recess which is of dish-shaped design, a projection extending from the center of the piston recess in the direction of the injection nozzle. Owing to the dish-like basic shape, no narrow radii on the surface or cross-sectional jumps in the piston head occur in the piston recess, so that, when the internal combustion engine is in operation, if fuel droplets impinge onto the recess, these evaporate quickly.
According to a further refinement of the invention, the piston recess has, starting from the piston head, first, a flat entry with a low curvature and, from the region of the maximum recess depth, a greater curvature extending into the piston recess projection. This prevents an accretion of fuel in the region of the piston head and thus achieves a minimization of exhaust gas emissions. Preferably, the piston recess projection has a cone angle in a range of 90° to 160°.
Other objects, advantages, and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, for example.